The invention relates to a method and an apparatus for testing the leak-tightness of a fuel tank in a vehicle in which the internal pressure inside the fuel tank is detected.
Increasing environmental protection calls for measurements of the tightness of motor vehicle tanks or the leakage therefrom. A method and an apparatus for detecting the tank filling level according to DE 42 03 099 A1 therefore permits both sensing of the tank filling level and testing of the tank leak-tightness. For this purpose a differential pressure gauge is provided in the upper part of the tank. The known apparatus furthermore comprises a tank leak-tightness testing device, a gas emission test device for testing whether the fuel in the tank is gassing, an evacuation/drainage control device in order to subject the tank to a variation, a gradient determining device for determining the value of pressure change gradient from at least one pressure change and a time span associated therewith.
Use is made of sensor signals from a differential pressure sensor, which is already provided for performing a reliability check on the functioning of a tank venting system interacting with the tank in the motor vehicle. In this case, however, the filling level is determined only indirectly from the pressure above the liquid level. The known method and the known device are therefore rather expensive and inaccurate. Furthermore, it is only possible to measure a filling level if the tank is leak-tight.
The object of the present invention is to further improve the testing of the leak-tightness of a fuel tank. According to the invention this object is achieved in that the internal pressure detected is evaluated by weighting this with a value obtained for a change in the speed of the vehicle.
The invention is based on the finding that pressure measurements in the fuel tank vary greatly as a function of whether the vehicle is stationary, moving at a constant speed or accelerating/decelerating.
An advantageous development of the method according to the invention consists of forming differential pressure values from minimum and maximum values for the internal pressure detected, and generating a fault message if the differential pressure does not reach a predetermined threshold value within a set time once the change in speed exceeds a predetermined value. This provides a reliable check in the event of a correspondingly large change in the speed of the vehicle.
In order to avoid unnecessary, frequently recurrent testing, the method according to the invention may provide for the inference of a test termination signal (Readyflag), if the pressure differential exceeds the threshold value within the set time.
In order also to permit testing of the pressure sensor when at constant speed or stationary, another development of the method according to the invention provides for the inference of a test termination signal also if the change in speed is below the predetermined value and the pressure differential exceeds a predetermined threshold value.
The method according to the invention preferably provides for running of the test each time the vehicle is started.
After inversion, the fault message obtained by means of the method according to the invention can also display the leak-tightness. Provision can be made for display of the fault message and/or its storage for further evaluation.
An apparatus according to the invention, in which a pressure sensor is provided in fuel tanks for detecting the internal pressure, it being possible to feed the output signal from the pressure sensor to a control device, is characterized in that a sensor is designed with the control device for detecting the change in the speed of the vehicle, and the control device is designed to weight the output signal from the pressure sensor with the change in speed.
At the same time, the control device is preferably designed to generate a fault message, in such a way that differential pressure values are formed from minimum and maximum values for the internal pressure detected, and a fault message is generated if the differential pressure does not reach a predetermined threshold value within a set time once the change in speed exceeds a predetermined value.
The change in speed can be detected by means of a tachometer and corresponding differentiation of the output signal in the control device or by means of an acceleration sensor.
In addition, the apparatus according to the invention may be designed in such a way that a test termination signal (Readyflag) is inferred, if the pressure differential exceeds the threshold value within the set time.
In order also to permit checking of the pressure sensor in the absence of any change in the speed of the vehicle, the apparatus according to the invention may include provision for the control device to be designed in such a way that a test termination signal is also inferred if the change in speed is below the predetermined value and the pressure differential exceeds a predetermined threshold value.
Means may be provided both for displaying the fault message and for storing the fault message for subsequent diagnosis.